In recent years, a water-absorbent resin has been widely used in various fields such as hygienic articles such as disposable diaper and sanitary articles; agricultural and horticultural materials such as water-retaining agents and soil conditioners; and industrial and construction materials such as water blocking agents and dewfall preventing agents. Among these fields, it is frequently used especially for hygienic articles such as disposable diaper and sanitary articles.
As the water-absorbent resins, there have been known, for example, hydrolysates of starch-acrylonitrile graftcopolymers, neutralized products of starch-acrylate graftcopolymers, saponified products of vinyl acetate-acrylic ester copolymers, partially neutralized products of polyacrylic acid, and the like.
Generally, properties desired for the water-absorbent resin include a large amount of water absorption, an excellent water-absorption rate, a high gel strength after water absorption, and the like. Particularly, properties desired for the water-absorbent resin used in an absorbent material applied to a hygienic material include, in addition to a large amount of water absorption, an excellent water-absorption rate, and a high gel strength after water absorption, an excellent water absorption capacity under pressure, an appropriate particle size, a narrow particle size distribution, a small amount of rewet of the absorbed substance to the external of the absorbent material, an excellent dispersibility of the absorbed substance to the internal of the absorbent material, and the like.
Further, in recent years, with an absorbent material for use in a hygienic material such as disposable diaper or sanitary napkin being thinner and a manufacturing line being speeded up, it has been desired for water-absorbent resin particles that the particles do not break down even being exposed to a high pressure or a strong collision which has not been assumed so far, in other words, a high particle strength.
For example, an absorbent material for disposal diaper is generally manufactured by vacuuming water-absorbent resin particles and fibrous pulps on a metal mesh to laminate into layers with mixing in the air, in an equipment called a drum former. Thereafter, the absorbent material was compressed by a roller press or the like to maintain its shape and increase strength. In the manufacture of a thin absorbent material, the absorbent material is compressed by a higher pressure than that of the conventional absorbent material, and additionally, the amount of the pulp used is lowered. Therefore, a high force is loaded to the water-absorbent resin particles, whereby a breakdown of the particles tends to take place.
Further, a speed-up of a manufacturing line of the absorbent material to increase in productivity progresses, so that, in the above drum former, a speed at which the water-absorbent resin particles collide to the metal mesh and a supporting plate at the periphery thereof is increased, and the water-absorbent resin particles receive a collision higher than ever before, whereby the particles also tend to be broken down.
When a breakdown of the water-absorbent resin particles takes place, water absorption property gets lower. Therefore, there have been desired the water-absorbent resin particles in which particle strength against collision is high and the water absorption property doesn't get lower.
The water-absorbent resin is mainly produced by subjecting a water-soluble ethylenically unsaturated monomer to a reversed phase suspension polymerization or an aqueous solution polymerization. However, there have been disadvantages in a conventional reversed phase suspension polymerization method that water-absorbent resin particles having a large particle size are less likely to be produced, and an appropriate particle size that is suitable for a hygiene material is less likely to be obtained.
Therefore, in order to obtain water-absorbent resin particles having a large particle size, some manufacturing techniques have been suggested. For example, there have been suggested, in production methods according to a reversed phase suspension polymerization, a method comprising producing water-absorbent resin particles according to a polymerization of the monomer for the first step, thereafter, cooling the particles, again adding a monomer to the polymerization reaction solution in which the polymer particles in the first step are suspended, in a state a surfactant is precipitated, and polymerizing the mixture, to give large water-absorbent resin particles (Patent Publication 1), a method comprising producing water-absorbent resin particles according to a polymerization of the monomer for the first step, thereafter again adding a monomer to the polymerization reaction solution in which the polymer particles are suspended, and polymerizing the mixture, to agglomerate the polymer particles in the first step (Patent Publication 2), a method comprising producing water-absorbent resin particles according to a polymerization of the monomer for the first step, thereafter adding a specific surfactant having an HLB of 7 or more to the polymerization reaction solution in which the polymer particles are suspended, then again adding a monomer thereto, and polymerizing the mixture, to give large water-absorbent resin particles (Patent Publication 3), a method comprising producing water-absorbent resin particles according to a polymerization of the monomer for the first step, thereafter again adding a monomer to the polymerization reaction solution in which the polymer particles are suspended, in the presence of inorganic powders, and polymerizing the mixture, to give large water-absorbent resin particles (Patent Publication 4), and the like.
According to the above techniques, water-absorbent resin having a large particle size was obtained but did not sufficiently satisfy an appropriate particle size, a narrow particle size distribution, a high particle strength, and an excellent water absorption capacity under pressure.    Patent Publication 1: Japanese Patent Laid-Open No. Hei 3-227301    Patent Publication 2: Japanese Patent Laid-Open No. Hei 5-17509    Patent Publication 3: Japanese Patent Laid-Open No. Hei 9-12613    Patent Publication 4: Japanese Patent Laid-Open No. Hei 9-77810